This invention relates to an instrument for marking a semi-finished opthalmic lens blank for a subsequent blocking operation, said marking serving to indicate the optical center, cylindrical axis, and prism axis of the lens.
Opthalmic lenses are generally made from blanks formed from glass or plastic. The convex front surface of the blank is already finished. The back, eye-side surface of the blank is generally concave. The further concave toric surfaces required under prescriptions for astigmatism are thereafter ground into the back surface in compliance with the prescription.
In preparation for the grinding operation, the convex surface of the blank is mounted upon a retaining member known as a lens block which adheres to the blank by virtue of contact adhesive, a vacuum cup, or poured-in-place low melting temperature alloy. The lens blank and block assembly is then accurately mounted in a lens generating apparatus such as disclosed in U.S. Pat. No. 4,769,953 wherein a prescriptive surface is ground into the back surface as a cylinder cut comprised of a base curve whose curvature is based upon the length of a swing arm that sweeps a rotating grinding wheel across the lens surface, and a cross curve whose curvature is dependent upon the radius of the wheel and its angle with respect to the lens. The base curve intersects the optical center of the lens, namely the center determined by the patient's interpupillary distance. The base and cross curves are accordingly perpendicular to each other. The axis upon which the base curve is centered is generally referred to as the cylindrical axis.
In certain opthalmic lens prescriptions wherein there is need to correct a vision problem known as diplopia, the lens thickness is tapered from the perimeter inward, producing an attendant gradient of magnifying power. Such adjustment of the lens thickness is generally accomplished by utilizing a wedge to space the block from the lens. The axis upon which the wedge is centered relative to the optical center of the lens is referred to as the prism axis.
Following the grinding step, the lens is further lapped and polished, and the perimeter is bevelled or otherwise modified to permit fitting into an eyeglass frame.
It is important that the block lie accurately mounted upon the lens blank to ensure precise formation of the final lens product. Devices have earlier been disclosed for placing removable ink marks upon the convex surface to indicate the requisite positioning of the block. Such devices however, have generally required several manipulative steps, causing relatively high costs in terms of the increased time required of the technicians who operate the marking devices.
In particular, earlier marking devices have generally required a first step wherein the lens blank is carefully positioned upon a transparent target reticle plate while viewed by an illumination system that projects light through the reticle and lens blank. In a second step, positional adjustments are made in marking pens. In a third step, the illumination system or portion thereof is displaced to a location where it will not interfere with the function of the marking pens. In a fourth step, the marking pens are caused to move in a vertical path to place marks upon the convex surface. In a fifth step, the apparatus is restored to its starting configuration for use upon the next lens blank. In many marking devices, further additional steps are required to separately locate and mark the optical center, cylinder axis, and prism axis. In some marking devices, the cylinder axis is defined by two separated dots disposed on opposite sides of a dot representing the optical center, the three dots lying on a straight line. However, it is found preferable for the purpose of accurate placement of the block to have lines instead of separated dots for establishing the cylinder axis.
It is accordingly an object of the present invention to provide apparatus for the rapid and accurate determination and marking of the optical center, cylinder axis, and prism axis of an opthalmic lens blank.
It is a further object of this invention to provide apparatus as in the foregoing object wherein the marking is produced on the finished front convex surface of said lens blank in a manner to facilitate precise attachment of a holding block.
It is a another object of the present invention to provide apparatus of the aforesaid nature which achieves said marking in a single step, and presents said cylinder axis and prism axis as lines radially oriented with respect to the optical center of the lens.
These objects and other objects and advantages of the invention will be apparent from the following description.